1. Field of the Invention
The present invention generally relates to spacecraft radio frequency receivers and more particularly to an improved receiver that uses a tone based open loop receiver system and an ultrastable oscillator.
2. Description of the Related Art
Existing deep space receivers must phase-lock to the uplink signal and generate a data clock for command reception. These processes limit the sensitivity of present-day receivers to about −157 dBm for carrier lock and −145 dBm for reception of the minimum bit rate of 7.8125 bps. The phase-locking processes in the carrier and data tracking loops drive these threshold levels. The invention described below uses an open loop receiver that avoids the sensitivity loss associated with the use of phase lock loops (PLLs) or frequency lock loops (FLLs) and also avoids the lock threshold and acquisition delay to the system that a phase-lock or frequency-lock system would introduce.
Within this application several publications are referenced by arabic numerals within parentheses. Full citations for these, and other, publications may be found at the end of the specification immediately preceding the claims. The disclosures of all these publications in their entireties are hereby expressly incorporated by reference into the present application for the purposes of indicating the background of the present invention and illustrating the state of the art. For example, two of the papers referenced below [1, 2] describe tone-based systems that are used in ground-based systems to monitor spacecraft. However, these systems are different than the system presented below, which uses a tone-based system in conjunction with other elements, such as a USO, open loop receiver, etc., to receive weak uplink command signals at a spacecraft.